Indian intelligence agencies received no warnings before the three bomb blasts that killed 18 people in Mumbai, the biggest attack since Pakistani-based militants rampaged through the financial hub in 2008, a top official said on Thursday.Suspicion however fell on the Indian Mujahideen, a shadowy home-grown militant group known for its city-to-city bombing campaigns using small explosive devices planted in restaurants, at bus stops and on busy streets."There was no intelligence regarding a militant attack in Mumbai. That is not a failure of intelligence agencies," Home Minister Palaniappan Chidambaram told a news conference."Know that perpetrators have attacked and have worked in a very very clandestine manner. Maybe it's a very small group, maybe they did not communicate with each other."He said it was too early to point the finger at a particular group, but said the "coordinated terror attacks" could be in retaliation to a number of plots recently stopped by police or the arrests, including from the Indian Mujahideen.